This invention relates to systems for ion implantation of workpieces such as semiconductor wafers and, more particularly, to a Faraday system for measuring ion current in an ion implantation system or other ion beam treatment system.
Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded in the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Ion implantation systems usually include an ion source for converting a gas or a solid material into a well-defined ion beam. The ion beam is mass analyzed to eliminate undesired ion species, is accelerated to a desired energy and is directed onto a target plane. The beam is distributed over the target area by beam scanning, by target movement or by a combination of beam scanning and target movement.
In the operation of an ion implantation system, it is usually necessary to measure the cumulative ion dose implanted in the semiconductor wafer and the dose uniformity over the surface area of the wafer. Typically, ion implants are specified in terms of ion species, ion energy and ion dose. Measurement of ion dose, at least at intervals during an ion implant, is necessary since ion sources typically do not deliver accurate, constant ion beam currents. Semiconductor fabrication processes typically require dose accuracies within 1%.
Cumulative ion dose is typically measured using a Faraday cup, or dose measurement cup, positioned in the vicinity of the target wafer. For example, the Faraday cup may be positioned adjacent to the semiconductor wafer, and the ion beam may be deflected to the Faraday cup at intervals during an ion implant. The Faraday cup may be an electrically conductive cup having an entrance aperture for receiving the ion beam. The ion beam generates in the Faraday cup an electrical current that is representative of ion beam current. The electrical current is supplied to an electronic dose processor, which integrates the current with respect to time to determine the cumulative ion dose. The dose processor may be part of a feedback loop that is used to control the ion implanter. For example, ion implantation may be terminated when a predetermined dose has been reached. The use of Faraday cups to measure dose and dose uniformity in ion implanters is described, for example, in U.S. Pat. No. 4,922,106 issued May 1, 1990 to Berrian et al. and U.S. Pat. No. 4,980,562 issued Dec. 25, 1990 to Berrian et al.
When energetic ions in the ion beam enter the Faraday cup and impinge on surfaces within the Faraday cup, a number of physical processes may occur. These processes include ion scattering, secondary particle emission (electrons, ions, neutrals and photons), surface sputtering and generation of tertiary electrons, which result from secondary particles impacting the cup body walls. A portion of the secondary and tertiary electrons may escape from the Faraday cup through its entrance aperture. Escape of electrons from the Faraday cup produces a measurement error.
One approach to inhibiting the escape of electrons from the Faraday system is the use of an electrostatic suppression electrode positioned at the entrance to the Faraday cup. By negatively charging the electrostatic suppression electrode, the escape of electrons from the Faraday cup is inhibited. A Faraday cage having an electrostatic suppression electrode is disclosed in U.S. Pat. No. 4,135,097 issued Jan. 16, 1979 to Forneris et al. U.S. Pat. No. 4,135,097 discloses an alternate configuration wherein a pair of magnets is used for electron suppression. A magnetically suppressed Faraday system is disclosed in U.S. Pat. No. 5,757,018 issued May 26, 1998 to Mack et al. In another prior art approach to inhibiting escape of electrons, the depth of the Faraday cup is made large in comparison with the width of its entrance aperture, so that electrons generated at the bottom of the Faraday cup have a relatively small probability of escaping through the entrance aperture.
Ion implanters are typically required to operate over a wide energy range. In particular, ion implanters are increasingly required to operate at low energies due to process requirements for shallow and ultra-shallow junctions. At low energies, the ion beam diameter increases significantly due to the well-known space charge effect. In order to perform accurate dose measurements at low ion energies, Faraday cups with large entrance apertures are therefore required. Since space considerations in the ion implanter may prevent a corresponding increase in the depth of the Faraday cup, the Faraday cup may have a relatively large entrance aperture width in comparison to the cup depth. This exacerbates the problem of secondary electron escape and results in reduced measurement accuracy.
Accordingly, there is a need for improved methods and apparatus for measuring ion beam current, particularly at low energies.
According to a first aspect of the invention, a Faraday system is provided for use in an ion beam treatment system. The Faraday system comprises a Faraday cup body defining a chamber which has an entrance aperture for receiving an ion beam, a suppression electrode positioned in proximity to the entrance aperture to produce electric fields for inhibiting escape of electrons from the chamber, and a magnet assembly positioned to produce magnetic fields for inhibiting escape of electrons from the chamber.
In one embodiment, the ratio of chamber depth to entrance aperture width is less than 2.0. In another embodiment, the ratio of chamber depth to entrance aperture width is less than 1.0.
The magnet assembly may comprise first and second magnets disposed on opposite sides of the Faraday cup body.
The suppression electrode may comprise a suppression ring disposed around the entrance aperture. The suppression ring may have an aperture that is larger in width than the entrance aperture. The suppression ring may be tapered toward the aperture in the suppression ring. The Faraday system may further comprise a suppression power supply for biasing the suppression electrode at a suppression voltage relative to the Faraday cup body.
In other embodiments, all or part of an interior surface of the Faraday cup body is coated with a material that exhibits relatively low electron emission. In one example, the interior surface of the Faraday cup body has a carbon coating. In other embodiments, the Faraday cup body is fabricated of graphite.
In other embodiments, the Faraday cup body includes a grid of holes facing the entrance aperture. The grid of holes may be located at the bottom of the Faraday cup opposite the entrance aperture.